Pseudo stereophonic device

ABSTRACT

In a pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal, there are provided m delay units connected in series and gradually delaying an input signal S, m FIR digital filters for respectively subjecting output signals S k  (k=1, 2, . . . m) of the delay units to filter processing, and an operating circuit for executing a predetermined operation on the basis of outputs Y k  (k=1, 2, . . . m) of the respective FIR digital filters, to produce pseudo stereophonic signals L OUT  and R OUT .

TECHNICAL FIELD

The present invention relates generally to a pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal.

BACKGROUND OF THE INVENTION

Examples of a pseudo stereophonic method for producing a pseudo stereophonic signal from a monophonic signal mainly include two methods; a comb filter system and a band division system.

(1) Comb Filter System

FIG. 5 illustrates the configuration of a pseudo stereophonic device employing the comb filter system.

The pseudo stereophonic device employing the comb filter system has the simplest configuration as a pseudo stereophonic device.

An input signal S is fed to a first adder 111 and a second adder 112, and is fed to a delay unit 101. A signal obtained by delaying the signal S in the delay unit 101 is fed to a multiplier 102, where the signal is multiplexed by a predetermined factor. An output of the multiplier 102 is fed to the first adder 111 and the second adder 112.

In the first adder 111, the output signal of the multiplier 102 is added to the input signal S, and the result of the addition is outputted as a pseudo left signal L_(OUT). In the second adder 112, the output signal of the multiplier 102 is subtracted from the input signal S, and the result of the subtraction is outputted as a pseudo right signal R_(OUT).

The longer a delay time allowed to the delay unit 101 is, the more a stereophonic feeling between the two output signals L_(OUT) and R_(OUT) is increased. However, the signal obtained by the delay is heard as an echo. Accordingly, a delay time of several microseconds is generally allowed to the delay unit 101.

If the delay time of the delay unit 101 is several microseconds, however, non-correlation between two channels is insufficient, so that the stereophonic feeling is insufficient. Particularly, the comb filter system is not suitable for two-channel reproduction processing of a multichannel signal using a sound image localization processing technique.

(2) Band Division System

FIG. 6 illustrates the configuration of a pseudo stereophonic device employing the band division system.

An input signal S is delayed by one sampling time period by each of a plurality of delay units D₁ to D_(m) connected in series.

Pairs of multipliers ML₁ and MR₁ to ML_(m+1) and MR_(m+1) are respectively provided with respect to the input signal S and output signals of the delay units D₁ to D_(m). The input signal S and each of the output signals of the delay units D₁ to D_(m) are inputted to the corresponding pair of multipliers, where they are multiplexed by a factor.

Output signals of the one multipliers ML₁ to ML_(m+1) in the pairs of multipliers are added to each other by adders AL₁ to AL_(m), and the result of the addition is outputted as a pseudo left signal L_(OUT). Output signals of the other multipliers MR₁ to MR_(m+1) in the pairs of multipliers are added to each other by adders AR₁ to AR_(m), and the result of the addition is outputted as a pseudo right signal R_(OUT).

The delay units D₁ to D_(m), the one multipliers ML₁ to ML_(m+1) in the pairs of multipliers, and the adders AL₁ to AL_(m) constitute a first FIR (Finite Impulse Response) digital filter.

The delay units D₁ to D_(m), the other multipliers MR₁ to MR_(m+1) in the pairs of multipliers, and the adders AR₁ to AR_(m) constitute a second FIR digital filter. The delay units D₁ to D_(m) are shared between the first FIR digital filter and the second FIR digital filter.

The filter characteristics of the first FIR digital filter are shown in FIG. 7, and the filter characteristics of the second FIR digital filter are shown in FIG. 8. As can be seen from FIGS. 7 and 8, the filter characteristics of each of the FIR digital filters are such characteristics that a frequency band is divided into a plurality of pass and stop bands, and the pass bands and the stop bands alternately appear. The filter characteristics are such characteristics that the pass and stop bands in the first FIR digital filter and the pass and stop bands in the second FIR digital filter are opposite to each other such that the respective filter outputs L_(OUT) and R_(OUT) are not correlated with each other.

In the pseudo stereophonic device employing the band division system, if each of the pass and stop bands in each of the FIR digital filters is wide, the FIR digital filter may be only composed of hundreds of taps. However, sound is offset for each wide frequency band, so that an unnatural tone color is obtained. On the other hand, if each of the pass and stop bands in each of the FIR digital filters is narrowed, non-correlation is improved, so that a natural tone color is obtained. However, the FIR digital filter must be composed of not less than thousands of taps, so that a huge amount of processing is required.

As described above, in the pseudo stereophonic device employing the comb filter system, the processing is light, while sufficient non-correlation (stereophony) cannot be performed. In the pseudo stereophonic device employing the band division system, a huge amount of processing is required to perform sufficient non-correlation.

An object of the present invention is to provide a pseudo stereophonic device in which sufficient non-correlation can be performed, and a huge amount of processing is not required.

DISCLOSURE OF INVENTION

In a pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal, a first pseudo stereophonic device according to the present invention is characterized by comprising m delay units connected in series and gradually delaying an input signal S, m FIR digital filters for respectively subjecting output signals S_(k) (k=1, 2, . . . m) of the delay units to filter processing, and an operating circuit for executing, letting Y_(k) (k=1, 2, . . . m) be outputs of the respective FIR digital filters, an operation expressed by the following equation (1), to produce pseudo stereophonic signals L_(OUT) and R_(OUT): $\begin{matrix} {{L_{OUT} = {Y_{1} + {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}{R_{OUT} = {Y_{1} - {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}} & (1) \end{matrix}$

The delay unit in the first row may be omitted, and the input signal S may be inputted to the FIR digital filter in the first row and the delay unit in the second row.

Letting n_(k) be the number of taps composing the FIR digital filter in the k-th row, it is preferable that a filter factor of each of the FIR digital filters satisfies the condition expressed by the following equation (2):

W _(k,i) =W _(m−k+2,n) _(m) _(−j+1)  (b).

A second pseudo stereophonic device according to the present invention is a pseudo stereophonic device equivalent to the first pseudo stereophonic device satisfying the foregoing equation (2), characterized in that one multiplier is shared between two multipliers, respectively having equal filter factors, in the different FIR digital filters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram showing the configuration of a pseudo stereophonic device according to a first embodiment of the present invention;

FIG. 1B is a circuit diagram showing the configuration of a pseudo stereophonic device according to an alternate embodiment of the present invention;

FIG. 2 is a circuit diagram showing the configuration of a pseudo stereophonic device according to a second embodiment of the present invention;

FIG. 3 is a circuit diagram showing the configuration of a pseudo stereophonic device according to a third embodiment of the present invention;

FIG. 4 is a block diagram showing an applied example;

FIG. 5 is a circuit diagram showing the configuration of a pseudo stereophonic device employing a comb filter system;

FIG. 6 is a circuit diagram showing the configuration of a pseudo stereophonic device employing a band division system;

FIG. 7 is a characteristic view showing filter characteristics of a first FIR digital filter in the pseudo stereophonic device employing the band division system shown in FIG. 6; and

FIG. 8 is a characteristic view showing filter characteristics of a second FIR digital filter in the pseudo stereophonic device employing the band division system shown in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1 to 4, embodiments of the present invention will be described.

[1] Description of First Embodiment

FIG. 1A illustrates the configuration of a pseudo stereophonic device.

The pseudo stereophonic device has a hybrid configuration comprising a combination of a comb filter system and FIR digital filters.

A monophonic input signal S is delayed by a predetermined time period by each of a plurality of delay units D_(k,1) (k=1, 2, . . . m) (where m is an odd number) connected in series.

Output signals of the delay units D_(1, 1) to D_(m, 1) are respectively fed to FIR digital filters F_(k) (k=1, 2, . . . m), where they are subjected to filter processing.

Each of the FIR digital filters F₁ to F_(m) is constituted by a plurality of delay units whose delay time is one sampling time period, a plurality of multipliers, and a plurality of adders, as is well known.

The delay units are respectively indicated by D_(k, j) (k=1, 2, . . . m:j=2, 3, . . . n_(k)). The multipliers are respectively indicated by M_(k, j) (k=1, 2, . . . m:j=1, 2, . . . n_(k)). The adders are respectively indicated by A_(k, j) (k=1, 2, . . . m:j=2, 3, . . . n_(k)) n_(k) indicates the number of taps composing the FIR digital filter in the k row.

The FIR digital filters F₁ to F_(m) respectively have filter factors W_(kj) (k=1, 2, . . . m:j=1, 2, . . . n_(k)) indicated by the multipliers M_(kj) (k=1, 2, . . . M:j=1, 2, . . . n_(k)) included therein.

The results of the filter processing by the FIR digital filters F₁ to F_(m) are respectively taken as Y_(k) ( k=1, 2, . . . m).

The results of the filter processing Y_(k) (k=2, 3, . . . m) by the FIR digital filters F₂ to F_(m) other than the FIR digital filter F₁ in the first row are added to each other by the plurality of adders B₃ to B_(m), and the result of the addition is outputted from the adder B₃. The adder B₁ adds the output of the adder B₃ and the result of the filter processing Y₁ by the FIR digital filter F₁ in the first row to each other, and outputs the result of the addition as a pseudo left signal L_(OUT).

The adder B₂ subtracts the output of the adder B₃ from the result of the filter processing Y₁ by the FIR digital filter F₁ in the first row, and outputs the result of the subtraction as a pseudo right signal R_(OUT).

The pseudo left signal L_(OUT) and the pseudo right signal R_(OUT) which are thus obtained are pseudo stereophonic signals. The pseudo stereophonic signal L_(OUT) and the pseudo stereophonic signal R_(OUT) are expressed by the following equation (3): $\begin{matrix} {{L_{OUT} = {Y_{1} + {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}{R_{OUT} = {Y_{1} - {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}} & (3) \end{matrix}$

In the pseudo stereophonic device, non-correlation processing in the comb filter system in which processing is light can be made the most of, and the FIR digital filters are employed only in a portion where the non-correlation by the comb filter system is insufficient. Accordingly, the number of taps composing the FIR digital filter can be significantly made smaller, as compared with the number of taps composing the FIR digital filter employed in the band division system.

FIG. 1B is a circuit diagram showing the configuration of a pseudo stereophonic device according to an alternate embodiment of the present invention. FIG. 1B is similar to the circuit diagram of FIG. 1A. However, in the circuit diagram of FIG. 1B, the delay unit D_(1,1) in the first row of FIG. 1A is omitted, and the input signal S is inputted to the FIR digital filter F₁ in the first row and the delay unit D_(2,1) in the second row.

[2] Description of Second Embodiment

FIG. 2 illustrates the configuration of a pseudo stereophonic device.

The pseudo stereophonic device corresponds to a case where m=3, n₁=1, n₂=n₃=5 in the pseudo stereophonic device shown in FIG. 1.

A monophonic input signal S is delayed by a predetermined time period by each of a plurality of three delay units D_(1, 1), D_(2, 1), and D_(3, 1) connected in series. Signals obtained by delaying the signal S in the delay units D_(1, 1), D_(2, 1), and D_(3, 1) are respectively taken as S₁, S₂, and S₃.

The output signal S₁ of the delay unit D_(1, 1) is fed to a first FIR digital filter F₁. The output signal S₂ of the delay unit D_(2, 1) is fed to a second FIR digital filter F₂. The output signal S₃ of the delay unit D_(3, 1) is fed to a third FIR digital filter F₃.

The first FIR digital filter F₁ is constituted by one multiplier M_(1, 1). That is, the first FIR digital filter F₁ is an FIR digital filter composed of one tap.

The second FIR digital filter F₂ is constituted by four delay units D_(2, 2) to D_(2, 5) whose delay time is one sampling time period, five multipliers M_(2, 1) to M_(2, 5), and four adders A_(2, 2) to A_(2, 5). That is, the second FIR digital filter F₂ is an FIR digital filter composed of five taps respectively having filter factors W_(2, 1) to W_(2, 5) indicated by the multipliers M_(2, 1) to M_(2, 5).

The third FIR digital filter F₃ is constituted by four delay units D_(3, 2) to D_(3, 5) whose delay time is one sampling time period, five multipliers M_(3, 1) to M_(3, 5) and four adders A_(3, 2) to A_(3, 5). That is, the third FIR digital filter F₃ is an FIR digital filter composed of five taps respectively having filter factors W_(3, 1) to W_(3, 5) indicated by the multipliers M_(3, 1) to W_(3, 5).

The result of filter processing Y₂ by the second FIR digital filter F₂ and the result of filter processing Y₃ by the third FIR digital filter F₃ are added to each other by an adder B₃.

An adder B₁ adds the result of filter processing Y₁ by the first FIR digital filter F₁ and the result of the addition (Y₂+Y₃) by the adder B₃ to each other, and outputs the result of the addition as a pseudo left signal L_(OUT).

An adder B₂ subtracts the result of the addition (Y₂+Y₃) by the adder B₃ from the result of filter processing Y₁ by the first FIR digital filter F₁, and outputs the result of the subtraction as a pseudo right signal R_(OUT).

Consequently, the pseudo stereophonic signals L_(OUT) and R_(OUT) are expressed by the following equation (4):

L _(OUT) =Y ₁ +Y ₂ +Y ₃

R _(OUT) =Y ₁ −Y ₂ −Y ₃  (4)

Considering that Y₁, Y₂, and Y₃ are common between L_(OUT) and R_(OUT), a pseudo stereophonic device can be substantially realized in an amount of processing performed by an FIR digital filter composed of approximately 10 taps. It is found that the pseudo stereophonic device in the above-mentioned embodiment is significantly decreased in the amount of processing, as compared with a pseudo stereophonic device employing a band division system which requires processing performed by an FIR digital filter composed of not less than thousands of taps. The acoustic effect is approximately the same as that in the pseudo stereophonic device employing the band division system.

[3] Description of Third Embodiment

It is preferable that in the second embodiment, the factors (filter factors) of the respective multipliers M_(2, 1) to M_(2, 5) in the second FIR digital filter F₂ and the factors (filter factors) of the respective multipliers M_(3, 1) to M_(3, 5) in the third FIR digital filter F₃ have the following relationships:

Factor of Multiplier M_(2,1)=Factor of Multiplier M_(3,5)

Factor of Multiplier M_(2,2)=Factor of Multiplier M_(3,4)

Factor of Multiplier M_(2,3)=Factor of Multiplier M_(3,3)

Factor of Multiplier M_(2,4)=Factor of Multiplier M_(3,2)

Factor of Multiplier M_(2,5)=Factor of Multiplier M_(3,1)

The following are specific examples:

Delay time of Delay unit D_(1,1): 7.48 [msec]

Delay time of Delay unit D_(2,1): 11.54 [msec]

Delay time of Delay unit D_(3,1): 27.32 [msec]

Factors of Multipliers M_(2,1),M_(3,5): 5.35406805574894e-2

Factors of Multipliers M_(2,2), M_(3,4): 1.596434861421585e-1

Factors of Multipliers M_(2,3), M_(3,3): 2.495117336511612e-1

Factors of Multipliers M_(2,4), M_(3,2): −1.586669087409973e-1

Factors of Multipliers M_(2,5), M_(3,1): −5.25641143321991e-2

The above-mentioned relationships of the filter factors among the FIR digital filters are expressed by the following general equation:

Letting n_(k) be the number of taps composing an FIR digital filter in the k-th row and M_(k,j) be the multipliers in the FIR digital filters F₂ to F_(m) (k=2, 3, . . . m;j=1, 2, . . . n_(k)), a factor W (k,j) of each of the multipliers M_(k,j) (a filter factor for the j-th tap (1≦j≦n_(k)) in the FIR digital filter in the k-th row (2≦k≦m)) may be set so as to satisfy the condition expressed by the following equation (5):

W _(k,j) =W _(m−k+2,n) _(m) _(−j+1)  (5)

In the pseudo stereophonic device shown in FIG. 2, when the filter factors are set so as to satisfy the condition expressed by the foregoing equation (5), the pseudo stereophonic device shown in FIG. 2 can be replaced with an equivalent circuit as shown in FIG. 3. In FIG. 3, portions corresponding to those shown in FIG. 2 are assigned the same reference numerals.

In the equivalent circuit, multipliers M_(2, 1) to M_(2, 5) shown in FIG. 3 are shared between the multipliers M_(2, 1) to M_(2, 5) and the multipliers M_(3, 5) to M_(3, 1), which respectively have the same factors, in the second FIR digital filter F₂ and the third FIR digital filter F₃ shown in FIG. 2.

The result of addition of an output S_(2, 1) of a delay unit D_(2, 1) and an output S_(3, 5) of a delay unit D_(3, 5) by an adder a₁ is fed to the multiplier M_(2, 1). The result of addition of an output S_(2, 2) of a delay unit D_(2, 2) and an output S_(3, 4) of a delay unit D_(3, 4) by an adder a₂ is fed to the multiplier M_(2, 2).

The result of addition of an output S_(2, 3) of a delay unit D_(2, 3) and an output S_(3, 3) of a delay unit D_(3, 3) by an adder a₃ is fed to the multiplier M_(2, 3). The result of addition of an output S_(2, 4) of a delay unit D_(2, 4) and an output S_(3, 2) of a delay unit D_(3, 2) by an adder a₄ is fed to the multiplier M_(2, 4). The result of addition of an output S_(2, 5) of a delay unit D_(2, 5) and an output S_(3, 1) of a delay unit D_(3, 1) by an adder a₅ is fed to the multiplier M_(2, 5).

Outputs of the multipliers M_(2, 1), M_(2, 2), M_(2, 3), M_(2, 4), and M_(2, 5) are added to each other by adders b₃ to b₆, and the result of the addition is outputted from the adder b₃. An adder b₁ adds an output Y₁ of the multiplier M_(1, 1) and the output of the adder b₃ to each other, and outputs the result of the addition as a pseudo left signal L_(OUT). An adder b₂ subtracts the output of the adder b₃ from the output Y₁ of the multiplier M_(1, 1) and outputs the result of the subtraction as a pseudo right signal R_(OUT).

Letting S_(k,j) (k=2, 3, . . . m:j=1, 2, . . . n_(k)) be outputs of delay units D_(k, j) (k=2, 3, . . . m:j=1, 2, . . . n_(k)), respectively, the pseudo stereophonic signals L_(OUT) and R_(OUT) are expressed by the following equation (6): $\begin{matrix} {{L_{OUT} = {Y_{1} + {\sum\limits_{k = 2}^{3}\quad {\sum\limits_{j = 1}^{5}\quad {W_{k,j}\quad \left( {S_{k,j} + S_{{5 - k},{6 - j}}} \right)}}}}}{R_{OUT} = {Y_{1} - {\sum\limits_{k = 2}^{3}\quad {\sum\limits_{j = 1}^{5}\quad {W_{k,j}\quad \left( {S_{k,j} + S_{{5 - k},{6 - j}}} \right)}}}}}} & (6) \end{matrix}$

According to the third embodiment, the number of operations can be made smaller, as compared with that in the above-mentioned second embodiment.

[4] Description of Applied Example

FIG. 4 illustrates an example in which the pseudo stereophonic device shown in FIGS. 1A, 1B, 2, or 3 is applied to such an acoustic device that a signal having three-channel (Left, Center, Right) signals at the front and a single-channel (Surround) signal at the rear, for example, a four-channel signal obtained by decoding a Dolby prologic looks as if it was outputted from a total of four speakers, i.e., right and left speakers and right and left speakers respectively arranged ahead of and behind a listener, although it was outputted from two speakers (a left speaker and a right speaker) arranged ahead of the listener.

The single-channel surround signal is inputted to the pseudo stereophonic device 10 shown in FIGS. 1A, 1B, 2, or 3. The pseudo stereophonic device 10 produces a pseudo surround left signal L_(OUT) and a pseudo surround right signal R_(OUT) from the single-channel surround signal.

The pseudo surround left signal L_(OUT) and the pseudo surround right signal R_(OUT) are fed to a sound image localization processor 20. The sound image localization processor 20 subjects the inputted signals L_(OUT) and R_(OUT) to sound image localization processing such that the inputted signals L_(OUT) and R_(OUT) are localized at the left rear and the right rear of the listener.

On the other hand, an adder 2 adds the left signal Left to a signal obtained by subjecting the center signal Center to gain control of −6 dB in a multiplier 1. Further, an adder 3 adds the right signal Right to a signal obtained by subjecting the center signal Center to gain control of −6 dB in the multiplier 1.

An output of the adder 2 and a surround left signal L_(OUT′) after the localization processing which is outputted from the sound image localization processor 20 are added to each other by an adder 4, and the result of the addition is taken as an output Lphantom to the left speaker. An output of the adder 3 and a surround right signal R_(OUT′) after the localization processing which is outputted from the sound image localization processor 20 are added to each other by an adder 5, and the result of the addition is taken as an output Rphantom to the right speaker. 

What is claimed is:
 1. A pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal, comprising: m delay devices connected in series and gradually delaying an input signal S; m FIR digital filters for respectively subjecting output signals S_(k) (k=1, 2, . . . m) of the delay devices to filter processing; and an operating circuit for executing, letting Y_(k) (k=1, 2, . . . m) be outputs of the respective FIR digital filters, an operation expressed by the following equation (a), to produce pseudo stereophonic signals L_(OUT) and R_(OUT): $\begin{matrix} {{L_{OUT} = {Y_{1} + {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}{R_{OUT} = {Y_{1} - {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}} & (a) \end{matrix}$

wherein m is an integer greater than or equal to
 2. 2. A pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal, comprising: m delay devices connected in series and gradually delaying an input signal S; m FIR digital filters for respectively subjecting output signals S_(k) (k=1, 2, . . . m) of the delay devices to filter processing; and an operating circuit for executing, letting Y_(k) (k=1, 2, . . . m) be outputs of the respective FIR digital filters, an operation expressed by the following equation (a), to produce pseudo stereophonic signals L_(out) and R_(OUT′); $\begin{matrix} {{L_{OUT} = {Y_{1} + {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}{R_{OUT} = {Y_{1} - {\sum\limits_{k = 2}^{m}\quad Y_{k}}}}} & (a) \end{matrix}$

wherein m is an integer greater than or equal to 2; a first delay device of said m delay devices is omitted such that the total number of delay devices is m−1, and the input signal S is input to a first FIR digital filter of said m FIR digital filters and a second delay device of said m delay devices.
 3. The pseudo stereophonic device according to either one of claims 1 and 2, wherein letting n_(k) be the number of taps composing an FIR digital filter in a k-th row (2≦k≦m) out of the m FIR digital filters, a j-th tap (1≦j≦n_(k)) in the FIR digital filter in the k-th row has a filter factor W (k,j), which satisfies the condition expressed by the following equation (b): W _(k,j) =W _(m−k+2,n) _(m) _(−j+1)  (b).
 4. A pseudo stereophonic device equivalent to the pseudo stereophonic device according to claim 3, wherein one multiplier is shared between two multipliers, respectively having equal filter factors, in the different FIR digital filters. 